Logging is the process of storing data about events occurring in a data processing system such that the data is usable for post-facto analysis of the events that generate that data. Such data is called log data, or simply log (plural: logs). A file that stores log data in a file-system is called a log file.
Some example uses of log data are in diagnostic, forensic, and auditing applications. For example, log data can be used for diagnosing a problem in the execution of a software product or a hardware component. As another example, log data can be used for investigating what went wrong or what was happening in a system at a given time. As another example, log data can be analyzed to ensure that a product or a product's operation in compliance with certain standards or requirements.
Log data can be voluminous. Some programs or components produce log data sporadically when exceptional events occur. Some other programs or components can produce substantial amounts of log data routinely or when certain events occur in a data processing system. In fact, voluminous log data has been known to fill up an entire file-system space, causing other applications or components in the data processing environment to experience trouble accessing the file-system.